It is known from GB 2159998B to detect and store a time varying reference characteristic such as stress wave activity produced in a body by a writing instrument during the generation of a piece of handwriting e.g. a signature.
A contemporaneously created time varying test characteristic, such as stress wave activity produced in a body by a writing instrument during the generation of the same piece of handwriting, is then compared with the stored time varying reference characteristic. The comparison determines whether the reference and test characteristics have a common origination e.g. whether the pieces of handwriting have a common author.
It would be desirable to convert the analogue time varying signals into the digital domain. This would provide improved error correction facilities which would in turn enable the transmission of the time varying characteristic over a communication channel.
To achieve this, it would be necessary to add an analogue to digital conversion step to the above described process. For example, a time varying reference characteristic such as a stress wave activity produced in a body by a writing instrument during the generation of a piece of handwriting would be detected and converted to a digital reference signal. The digital reference signal would then be stored. A contemporaneously created time varying test characteristic, such as stress wave activity produced in a body by a writing instrument during the generation of the same piece of handwriting, would be converted to a digital test signal. The digital test signal would then be compared with the stored digital reference signal. The comparison would determine whether the time varying reference and test characteristics have a common origination e.g. whether the pieces of handwriting have a common author.
Nyquist's sampling criterion dictates the sampling rates required to convert an analogue signal to a digital signal. The criterion states “A signal having no significant spectral components above a frequency fH Hz is specified completely by its value at uniform spacings, no more than 1/(2 fH) seconds apart”. This corresponds to fs ≧2fH (Hz) where fs is the sampling frequency and fH is the highest frequency component of the analogue time varying signal.
The analogue time varying signal described in GB 2159998B has a maximum frequency component typically in the range of 100 kHz. The analogue time varying signal should, according to Nyquist's sampling criterion, be sampled at greater than twice its frequency i.e. at a rate of at least 200 kHz. The digital representation of a 2 sec portion of the analogue time varying reference signal at a resolution of 32 bits would require 1.6 MB of memory and 400,000 samples. Consequently, if a substantial number of digital reference signals are to be stored together, significant storage capacity would be required. Furthermore, the comparison of the digital reference signal and the digital test signal at 400,000 points would be processor intensive, and may be difficult to perform in real time.
Nyquist's criterion is fundamental to digital electronics, so any reduction in bandwidth of the digital signal would traditionally have come from reducing the resolution to 8 or 16 bits.